Ecosystem Gas Exchange in a California Grassland: Seasonal Patterns and Implications for Scaling

Abstract

We used the eddy covariance technique to measure exchanges of water vapor, energy, and CO2 between California serpentine grassland and the atmosphere. Even though the system was built around an inexpensive, one—dimensional sonic anemometer and a closed—path CO2 analyzer, energy balance closure was accurate to +20% at a net radiation of 500 W/m2, and the spectra and cospectra indicated only modest information loss from incompletely resolved high—frequency turbulence. In the early and middle parts of the growing season, net radition, latent heat, and sensible heat all had similar diurnal dynamics, with latent heat accounting for °60% of the net radiation. Late in the growing season, energy dissipation by latent heat dropped dramatically, even though the vapor pressure gradient remained high. The factor, an index of the role of canopy conductance in regulating transpiration (Jarvis and McNaughton 1986), decreased from 0.8 early in the growing season (indicating predominant control of transpiration by net radiation) to 0.1 late in the growing season (indicating a shift to control of transpiration by canopy conductance and vapor pressure deficit). Canopy conductance was a linear function of the product of net photosynthesis and relative humidity, divided by the CO2 concentration, as predicted by Ball (1988). The slope of the relationship, however, was greater early in the growing season at other times. Whole—ecosystem carbon exchange rates were modest, with midday net photosynthesis reaching maximum values of 6—8 μmol°m—2.s—1 in early April. Diurnal variation in photosynthesis roughly paralleled variation in photosynthetically active photon flux density (PFD), but with daily maximum increasing with canopy development early in the growing season and decreasing with drought at the end of the growing season. Photosynthesis did not clearly saturate at high levels of PFD. Ecosystem dark respiration increased strongly (Q10 = 4.6) with increasing soil surface temperature. The efficiency with which absorbed radition was used in ecosystem photosynthesis, integrated over entire days, was 0.0115 + 0.0015 mol CO2/mol PFD (°20% of the values measured for healthy, single leaves under low light conditions) until late in the growing season, when the efficiency fell sharply. Using simple assumptions to extrapolate measurements from 11 d to the entire growing season, we estimate ecosystem annual gross primary production to be 11.1 mol CO2/m2, yielding an annual net primary production of °133 g biomass/m2. This is near the center of the range of published values from above—ground harvest studies.